Air bags which inflate when an automobile is involved in a collision to offer protection to a passenger are becoming increasingly common as automobile equipment. To date, air bags are most commonly installed to protect a driver, although passenger air bags are also known. Such driver air bags are usually mounted at the center of a vehicle steering wheel, the size and shape of the compressed air bag and its inflator being largely dictated by the requirements of this location. There are many examples of inflators described in the patent literature.
The air bag is connected to an inflator in such a way that propellant gases emitted from the inflator cause inflation of the bag. Inflators related to the present invention include a housing having a propellant chamber surrounded by a filter chamber. There is also usually an ignitor chamber centered in the propellant chamber. The ignitor chamber contains an ignition squib and ignitor material. Housing walls between the chambers contain apertures for passage of gases from chamber to chamber when the inflator is activated, but it is important that propellant and ignition materials be protected from contact with moisture for the materials to remain effective. Moisture reacts with propellant compounds currently in common use, such as sodium azide compositions, diminishing their potential as gas generators. Housing apertures leading to the propellant and ignitor chambers are thus sealed against moisture ingress, the seals being sufficiently weak to break when the inflator is activated. The outermost wall of the inflator housing also contains apertures and this portion of the housing is commonly referred to as a diffuser.
The inflator squib is connected to one or more sensors located, for example, in the front bumper of the vehicle. When the vehicle is in a collision that activates the sensor, an electrical impulse is sent to the squib for activation thereof. The ignitor material explodes generating heat and hot gases which flow into the propellant chamber to activate the propellant. When the propellant is so activated, it generates a large volume of gas which travels through the filter chamber and diffuser apertures to inflate the bag and offer protection to a vehicle passenger.
Inflator housings related to the present invention generally include a base and an upper diffuser housing. These are separate components which are assembled to contain propellant material during manufacture of the inflator.
A practical requirement of an inflator for use as part of a vehicle air bag system is that the inflator pass what is known as a "bonfire" test. The inflator housing must not fail so as to create a hazard when subjected to heat sufficient to cause its contents to explode. One approach, as with the present invention, is for a housing to be of sufficient strength that when subjected to such conditions it resists explosion so as reduce the hazard to flying inflator parts. Another approach is to create a housing failure path along which gases released on explosion of inflator contents can flow under such conditions. An example of this latter approach is given in U.S. Pat. No. 4,902,036 of Zander et al., which issued Feb. 20, 1990, the specification of which is incorporated herein by reference.
It is generally advantageous for an inflator to be as lightweight as possible. In this way the inflator contributes as little as possible to the weight of the automobile. Inflator housings of aluminum have thus been considered to be advantageous, at least in this respect, to stainless steel housings. Examples of inflator housings including aluminum components are given in U.S. Pat. Nos. 4,530,516 (Adams et al., Jul. 23, 1985), 4,547,342 (Adams et.al., Oct. 15, 1985), and 4,561,675 (Adams et al., Dec. 31, 1985), the specifications of which are incorporated herein by reference.
It is also advantageous for an inflator housing to be simple to manufacture. It is thus desirable to avoid components which must be threaded together, such threadings requiring machining, which is relatively expensive. Further, welding of housing components containing explosive materials during manufacture is considered disadvantageous as it is necessary to avoid significant heat flow to explosive materials within the housing. Manufacture of the inflator disclosed in U.S. Pat. No. 4,547,342, for example, involves an inertia welding operation. An aluminum retaining ring holds gas generant pellets in place to keep them separated from the spinning inflator base during the welding process.
It will further be appreciated that reliance on materials which are generally inert are prefered for inflator housings. It is thus considered disadvantageous to rely on rubber or other organic polymer gaskets as sealants, since such materials generally degrade, if slowly, with time.
A non-welded inflator unit for automobile air bags is described in U.S. Pat. No. 4,923,212 of Cuevas issued May 8, 1990, the specification of which is incorporated herein by reference. A diffuser member and closure plate are sealed by magnaforming a peripheral lip of the diffuser member over the outer circumferential edge of the closure plate.
Another a non-welded gas generator with a rolled spun lip is described in U.S. Pat. No. 4,907,819 of Cuevas issued Oct. 31, 1989, the specification of which is incorporated herein by reference. A diffuser member and closure plate are mechanically sealed by rolling a peripheral lip portion of the diffuser member of the outer circumference edge of the closure plate. A locking ring is inserted against a bottom surface of the closure member within the periphery of the rolled lip, the locking ring being configured to engage an inner terminal edge portion of the lip.